The release of chemicals into the atmosphere, often polluting the air, is of substantial concern. Thus, in the chemical industry as new products and processes are developed, a key factor is the environmental effect. One means of reducing chemical emissions is to develop solvent-free processes, and to require that chemicals do not evaporate during processing or from the final product.
Liquid coatings traditionally have been solvent-based processes. Liquid coating is the process of replacing the gas, typically air, contacting a substrate, usually a solid surface such as a film or web, with a layer of liquid. After the deposition of a coating, it may remain a liquid, it may be dried if it contains solids dissolved in a volatile liquid, leaving behind a solid and typically adherent layer, or it may be "cured" (i.e., polymerized) or in some other way solidified to a functional and typically adherent layer. Volatile solvents typically have been used during coating processes and then evaporated off leaving the desired composition, especially when thin coatings are desired.
The coating process typically is selected based on the desired coat height (i.e., the coating thickness). Continuous liquid coating techniques (such as roll, curtain, slot, slide, gravure, and the like and combinations thereof) are commonly preferred for applying a composition to a smooth substrate to a height of about 5 micrometers or more. See generally, Modern Coating and Drying Technology, E. preferred for applying a composition to a smooth substrate to a height of about 5 micrometers or more. See generally, Modern Coating and Drying Technology, E. Cohen and E. Gutoff, VCH Publishing, N.Y., 1992. Rough or "three-dimensional" surfaces are preferably coated by spray processes.
Traditionally, solvent-borne thin coatings, i.e., dry thickness below about 5 micrometers, have been coated onto substrates for use as a release coating, a primer, or an anti-static layer, while thicker coatings have been used for adhesives, or for the manufacture of coated abrasives, etc. Continuous liquid coating techniques may be used to apply thin coatings; however, the composition typically has been diluted with a large amount of a solvent that is later removed by evaporation, leaving behind the composition at the desired thickness. The uniformity and thickness of the dried final layer may be difficult to control especially on rough surfaces. The added solvent leads to higher material costs, preparation costs, and solvent removal costs. In addition, the solvents typically used may be hazardous to the environment.
For continuous liquid coating processes as the line speed of the coating is increased, the process can become unstable allowing air entrapment to occur at the region where the composition first meets the substrate. This region is usually referred to as the "coating bead." Fortunately, electrostatic assistance may be used to alleviate the air entrapment problem occurring between the coating bead and the substrate. However, not all compositions can be applied by electrostatic assistance methods. The composition must have a sufficient conductivity such that free ions can move within the composition when an electric field is applied. Then as a high differential of electrical potential is applied between the composition and the substrate, an electric field is produced in the composition which induces the ions of one polarity in the composition to move to the coating bead surface which is closest to the substrate. In some coaters (e.g., gravure) which do not have a single coating bead, the ions still move to the composition's surfaces (e.g., the composition's surfaces in the gravure cells) which are closest to the substrate. This "inductive charging" of the composition causes an electrostatic pressure at the coating bead surface which can change the shape of the coating bead and prevent air from coming between the coating bead and the substrate. Thus with electrostatic assistance, increased line speeds may be obtained while maintaining uniformity when performing continuous coating. Even with discrete gravure coating methods, electrostatic assistance allows for increased line speeds because the electrostatic pressure "pulls" the composition out of the gravure cells.
Solvent-borne thin coatings may also be applied by spray processes. Although spray coating may be used to apply a composition to a smooth substrate, it is particularly useful as a method of coating rough or three-dimensional objects and sheet-like webs with rough or three-dimensional surfaces. Electrostatic spray processes are commonly preferred for applying a composition having a solvent to a rough surface to a coat height of 5 micrometers or more. However, a problem associated with spray processes is over-spraying (i.e., 50 to 80 percent of the composition may not reach the substrate). (Miller, E. P., Chapter 11, Electrostatic Coating; in Electrostatics and Its Applications, Wiley-Interscience(1973) Editor: A. D. Moore). Electrostatic spray processes provide a more controlled means of spraying, and thus reduce material loss.
In the more efficient electrostatic spray processes, the droplets are charged during formation using inductive-charging. Inductive-charging places a charge on the droplets through the electric field within the composition at the sprayer by which the electric field moves the positive free ions in opposite direction to the negative free ions. The excess of one polarity of ions accumulates at a region along the composition surface and creates the electrostatic pressure required to break the composition into a charged droplet mist. To achieve this inductive charging, the composition must have sufficient conductivity to ensure a reasonable number of free ions are present. Droplets in electrostatic spray coating typically range in diameter from about 50 micrometers (.mu.m) to about 200 .mu.m, whereas conventional (non-electrostatic) spray processes can have droplets as large as 500 .mu.m.
Electrospray, a distinct sub-class within electrostatic spraying, is restricted to low flow rates, which makes it useful for applying coatings to a thickness from about 0.005 micrometers to about 10 micrometers. Electrospray may be used to apply a thin coating without a solvent. In an electrospray process, the electrostatic pressure on the composition surface at the sprayhead causes a precisely controlled formation of one or more cones of composition from which a fine filament of liquid emanates. Each filament breaks up into a mist of droplets with droplet diameters on the order of the diameter of the filament. The diameter of the droplet can be controlled by the conductivity of the coating solution. Droplet diameters are typically less than 50 .mu.m, and can be less than 1 .mu.m if the conductivity is sufficiently large.
Although the electrospray process is an effective means of applying a thin coating, not every composition can be electrosprayed. As is the case with all electrostatic assistance methods, the composition must meet certain processing requirements. The viscosity and conductivity requirements for the composition to be coated vary with the electrostatic assistance method and with the coating thickness desired. For electrospray, the composition must be essentially either a single phase solution or a non-ionically-stabilized dispersion or emulsion, otherwise the composition may become unstable during the electrospray process. In a single phase solution ("true solution"), each component is completely soluble.
Compositions can be electrosprayed with or without a solvent, provided the composition is either a single phase solution or a non-ionically-stabilized emulsion or dispersion. Often a solvent must also be added to the composition in order to obtain the requisite component solubility. This added solvent, particularly if organic, may present environmental problems if it evaporates during or after processing and is not captured.
When a composition is truly solvent-free, substantially all of the initial components are present in some form in the final cured product. Thin coatings exist which are solvent-cast, but do not fit this definition because the solvent evaporates off during processing. For example, although ethanol or methanol can be added to electrosprayable compositions to enhance solubility and conductivity, they evaporate during processing.
Water-based compositions, although sometimes termed "solvent-free," cannot undergo cationic polymerization, at least until thoroughly dry.
A solvent can be added to a composition to enhance conductivity. To achieve the desired conductivity range, compositions often contain a polar solvent typically considered to be a volatile organic compound ("VOC"), in addition to a conductivity enhancer, i.e., salt. These volatile organic compounds can be hazardous to the environment.
Quaternary ammonium salts have been added to printing inks to enhance conductivity. However, depending on the choice of anion, these salts may not be compatible with cationic polymerization. U.S. Pat. No. 4,059,444 discloses adding quaternary ammonium salts, having anions with relatively low molecular weights such as sulfate, borate, and iodide, to ink. These conductivity control agents are added at levels of 0.05 to about 1 weight percent to increase the conductivity of electrostatically applied inks.
U.S. Pat. No. 4,303,924 discloses adding an oil-soluble salt, such as the mineral acid and organic acid quaternary salts of the Group Va elements, to a curable printing ink containing 0 to 30% of a polar organic solvent. All examples include a polar organic solvent.
To electrospray a thin layer having uniform thickness, each droplet from the electrospray mist must have a sufficiently low viscosity to allow for reasonable spreading on the substrate. However, for some applications it may be desirable to cure individual droplets on the substrate, e.g., slip sheets. Solvents and reactive diluents have been added to control viscosity. For example EPO Appln No. 93.924905.8 (Leir et al.) discloses adding reactive diluents to adjust viscosity for a cationically co-polymerizable polysiloxane release coating capable of being electrosprayed.
Regardless of the method of applying a coating to a substrate, the components preferably do not detrimentally interfere with the final performance of the product. A component preferably evaporates or does not interfere with polymerization or becomes physically trapped in the coating during processing, otherwise the component may migrate into the substrate and detrimentally affect the product's performance. Alternatively, it may later evaporate polluting the environment, or may later contact another surface, rub off, and contaminate that surface. To utilize the advantages electrostatic assistance methods offer, the compositions must have sufficient conductivity. Thus, the need exists for coating compositions capable of being applied by electrostatic assistance (i.e., electrostatically assisted continuous liquid coating (roll, curtain, slot, slide, gravure, and the like), electrostatic spray coating, or electrospray coating) where substantially all of the components are present in the final product and either co-polymerize with the other components or otherwise become a permanent part of the coating.